Molten metal, particularly steel, is continuously cast by being poured through the nozzle of either a ladle or an intervening tundish into the top of an open topped continuous casting mold having cooled side walls and containing a body of previously poured molten metal, the metal descending through the mold while solidifying against the mold's side walls so as to form a solidified skin containing unsolidified metal and producing a continuously descending cast strand, the mold having an open bottom through which the strand travels downwardly with its skin still containing some of the unsolidified metal until at some distance below the mold the strand completely solidifies throughout. Thereafter, the strand is cut to lengths which are inspected for surface defects which must be removed by chipping, milling, etc., as required for reheating and rolling of the lengths.
The skin solidifies initially in the upper portion of the mold and gradually increases in thickness with downward movement of the strand forming in the mold, thus forming the strand with an interior representing a sump containing molten metal until at the point where the strand completely solidifies throughout this sump is terminated by a resulting solid front.
The metal poured from the nozzle of either the ladle or the intervening tundish unavoidably contains particles of slag. If the top surface of the molten metal body within the mold is relatively static, it possibly cools so it solidifies enough to form particles of solidified metal. The molten metal necessarily poured from a height above the mold's top is in the form of a stream having at the mold's top portion a velocity typically in the order of from 1 to 1.5 m/sec. The result is that the stream has enough momentum to penetrate the body of molten metal in the mold for substantial distances before losing velocity to a degree where the stream blends into the body of molten metal.
If the molten metal enters the body of unsolidified metal in the mold centrally in the form of a vertical stream, it can possibly penetrate as an internal stream not only the unsolidified metal body in the mold itself but also the unsolidified metal in the sump below the mold where the skin's walls are converging towards each other. The molten metal may be poured from a tundish via a pipe having a lower end submerged in the body of unsolidified metal in the mold and having an open bottom so that the metal is in effect injected as a vertical downward stream, particularly when the mold is contoured to cast a strand of billet or bloom cross section of generally square shape. In the case of a mold having a slab cross section with a width very substantially greater than the thickness, this casting pipe may have a closed bottom and oppositely positioned outlets pointing towards the narrow side walls of the mold, in which case the stream is split into two separate streams traveling towards those narrow walls internally within the body of unsolidified metal in the mold.
It follows that the particles of slag can be via the stream carried to the skin forming within the mold, or possibly adjacently below the mold, so that the particles are entrapped by the solidifying skin-forming metal where the particles remain after the strand completely solidifies so as to form the solid front. Particularly when the two streams are formed by the casting pipe having the opposite side openings, the particles can be driven more or less directly towards and into the solidifying skin inside of the mold. Characteristically the bottom of the casting pipe is positioned not very far below the level of the body of molten metal in the mold so the particles are carried by the streams into the portion of the skin where it is just beginning to form by sollidification and is therefore relatively thin, thus causing the particles to be contained by the finally solidified strand on or near its surfaces. Particles of metal inadvertently solidified at the surface of the molten body in the mold may be drawn downwardly into the forming skin in the mold.
Semi-finished products cut from a solidified strand having a surface containing such particles as surface defects requires processing by undesirably extensive chipping, milling, etc., to remove the defects prior to reheating and rolling. This undesirably adds to the cost of making the final product.
Below the mold and above the solid front of the strand, it is possible to stir the unsolidified metal in the strand by using a multi-phase AC magnetic stirrer positioned outside of the strand so as to induce a traveling multi-phase field in the unsolidified metal which stirs the metal and distributes slag and possibly other particles uniformly so they do not concentrate at any location. It is also possible to use such a stirrer on the outside of the mold itself to in this way stir the molten metal inside of the mold so as to prevent the particles from becoming entrapped by the solidifying skin in the mold. However, a continuous casting mold must have thick water-cooled walls made of heavy copper plates so as to remove the heat from the molten metal and solidify a skin of adequate thickness before the forming strand leaves the mold. The mold walls may characteristically have a thickness of up to 80 mm, and although not solid, these walls make it very difficult for a multi-phase AC traveling field to penetrate them so as to be effective inside of the mold. For example, the effective penetrating field from a typical multi-phase AC stirrer operating even at the low frequency of 1.5 Hz is only from 50-60 mm through solid copper.
It is apparent that the continuous casting art needs some more effective means for splitting up or stirring within the mold the stream or streams of molten metal injected into the body of molten metal to maintain its volume while it with its forming skin is descending through and from the mold.